1. Field of the Invention
The present invention is directed to communication systems. More particularly, the invention is directed to receivers in wireless communication systems. Even more particularly, the present invention is directed to automatic gain control systems for such wireless communication system receivers.
2. Background of the Related Art
The use of receivers in wireless systems such as radio and cellular communication systems is well-known in the art. FIG. 1 shows a typical superheterodyne receiver design 10. Here, a radio frequency (RF) signal is received on antenna 15 and provided to RF amplifier 20. The RF signal is amplified by the RF amplifier 20 and in mixer 25 mixed with a signal from a local oscillator 30. This produces an intermediate frequency (IF) signal that is amplified in an IF amplifier 35 and filtered in a bandpass filter 40. The filtered IF signal is again amplified by an IF amplifier 45 and mixed in a product detector 50 with a signal from a beat frequency oscillator 55. The result is a signal that is amplified by a baseband amplifier 60 and digitized for further processing by an analog-to-digital (A/D) converter 65.
In such receivers, less amplifier gain is needed for strong signals, and it is important that a very strong signal not be amplified to the point that when amplified it distorts received information signals, overloads system components and possibly damages the components. For this reason, receivers typically have some sort of automatic gain control (AGC) system which controls one or more of the system amplifiers 20, 35, 45 and 60 to maintain the amplified signals within certain ranges (this control may be, e.g., through a bias applied to the amplifiers). In FIG. 1, the AGC unit 70 receives an IF input output by the IF amplifier 45 and uses it to generate bias signals controlling the RF amplifier 20 and the IF amplifiers 35 and 45.
An embodiment of the present invention provides an automatic gain control system for a wireless receiver that quickly differentiates desired in-band signals from high power out-of-band signals that overlap into the target band. The system measures power before and after passing a received signal through a number of filters that largely restrict the signal""s power to that which is in-band. By comparing the in-band energy of the received signal after filtering to the total signal energy prior to filtering, it is possible to determine whether a new in-band signal has arrived. The presence of this new in-band signal is then verified by a multi-threshold comparison of the normalized self-correlation to verify the presence of a new, desired in-band signal.